Method of mitigating sulfide trees in polyolefin insulated conductors

ABSTRACT

In an electric cable provided with a plastic sheath and an insulating layer consisting of polyolefin series resin applied directly or with the aid of other insulating layer on the copper conductor, polyolefin-series resin insulated electric cable provided in a desired position with a sulfide-capture layer consisting of a polyolefin-series resin composition incorporated with such powder of metals, salts of the metals, or the mixture thereof as to form water-insoluble metal sulfides by reacting with water soluble sulfides.

United States Patent Hasebe et al.

[151 3,663,742 1 May 16, 1972 [54] METHOD OF MITIGATING SULFIDE TREES INPOLYOLEFIN INSULATED CONDUCTORS [72] Inventors: Morikuni Hasebe; HiroshiNagai; Teruo Fukuda, all of Yokohama, Japan [73] Assignee: The FurukawaElectric Co., Ltd., Tokyo,

Japan [22] Filed: Oct. 2, 1970 [21] Appl. No.: 77,685

[30] Foreign Application Priority Data Oct. 6, 1969 Japan ..44/79338[52] U.S.Cl ..l74/l20SC,174/110SR,174/116 [51] Int. Cl. ..H01b 7/02 [58]Field ofSearch ..174/1 10, 120 SR, 120 SC, 102 SC, 174/105 SC, 106 SC,120 C, 112, 116

[56] References Cited UNlTED STATES PATENTS 2,930,838 3/1960 Chizalletet al. ..174/l20 SC INSULATING LAYER COPPER CONDUCTOR 2,294,919 9/1942Lunsford ..174/120 SC 2,754,353 7/1956 Gilliam ..174/12O SC 3,378,6284/1968 Gamer ..174/1l2 3,287,489 11/1966 l-lvizd, Jr. 1 74/102 SC FORElGN PATENTS OR APPLICATIONS 713,174 8/1954 Great Britain 174/1 10 PM836,255 6/1960 Great Britain..., .4 174/1 12 824,861 12/1959 GreatBritain 1 74/120 SC Primary Examiner-Lewis H Myers Assistant Examiner-A.T. Grimley Attorney-Kemon, Palmer & Estabrook [5 7] ABSTRACT In anelectric cable provided with a plastic sheath and an insulating layerconsisting of polyolefin series resin applied directly or with the aidof other insulating layer on the copper conductor, polyolefin-seriesresin insulated electric cable provided in a desired position with asulfide-capture layer consisting of a polyolefin-series resincomposition incorporated with such powder of metals, salts of themetals, or the mixture thereof as to form water-insoluble metal sulfidesby reacting with water soluble sulfides.

10 Claims, 5 Drawing Figures Patented May 16, 1972 3,663,742

2 She0ts-$he0t 1 FIG. i FIG. 2

INSULATING INSULATING LAYER LAYER 01 COPPER CONDUCTOR C2B1 rfl SULFIDELAYER {O3 {O3 CAPTURE COPPER LAYER CONDUCTOR PLASTIC SHEATH Fl 6 3PLASTIC SHEATH COPPER CONDUCTOR iOI \Q/ 01 i03 1 {O2 x INSULATING LAYERSULFIDE CAPTURE JUTE FILLER 3}}, 05 LAYER lOi PLASTIC SHEATH E F G. 4

.U) (1) Lu Z X 2 I i 4 9 16 2 5 36 DAYS OF IMMERSION OF SAMPLES INAMMONIUM SULFIDE Patented May 16, 1972 3,663,742

' 2 Sheets-Sheet 2 THICKNESS OF BLACK LAYER mm i 4 {a g 25 36 DAYS OFIMMERSION OF SAMPLES IN HYDROGEN SULFIDE METHOD OF MITIGATING SULFIDETREES IN POLYOLEFIN INSULATED CONDUCTORS This invention relates topolyolefin-series resin insulated electric cables and, moreparticularly, to polyolefin-series resin insulated electric cablesprovided with a means of preventing the deterioration of thepolyolefin-series resin insulation due to chemical trees (hereinafterreferred to as sulfide trees).

Polyolefin-series resins (including polymers of an olefin, copolymers oftwo or more olefins and cross-linked polyolefins prepared by using anorganic peroxide as a crosslinking agent) have inherently excellentelectric insulating and anti-chemical properties. Accordingly, they havebeen extensively used as the electric insulating material for electriccables. However, it has recently been found that the polyolefin seriesresin insulated electric cables laid in chemical plants and in wateroften result in unexpected dielectric breakdown. Detailed examination ofsuch troubled cables reveals that copper sulfide and copper oxide growin tree-like form within the polyolefin-series resin insulation toconstitute leakage paths (these tree-like leakage paths are hereinafterreferred to as sulfide trees to distinguish them from the so-calledtrees, which are electrically created in the insulation in a highelectric field). As result of extensive investigations conducted by theinventors about the formation of the sulfide trees, ithas been verifiedthat the sulfide trees will result if the polyolefinseries resininsulated electric cable is located at places where the water solublesulfide will be produced. It frequently occurs in the pits of chemicalplants that thecable is unintentionally exposed to the water-solublesulfidev either in gase or aqueous solution. Hydrogen sulfide isproduced through the luxuriation of zostera on the sea-bed. It is alsoproduced in the presence of sulfate-reducing bacteria. In thesesurroundings, hydrogen sulfide or sulfur ions will gradually penetrateby the aid of and together with water through the cable sheath and theinsulator of the cable core to reach the periphery of the copperconductor, where the hydrogen sulfide or sulfur ions will react withcopperconductor to produce the ,water insoluble copper sulfide. Thecopper sulfide thus produced will sometimes be oxidized to producecopper oxide.'As copper sulfide is crystallizable, it grows as tree-likecrystals within the polyolefin-series resin insulation at certaintemperatures. The growth rate is usually slow, but will be acceleratedaccording to the electric field and crystal structure of polymer. Thecopper sulfide trees thus grown eventually penetrate the entirethickness of the insulation and cause the dielectric breakdown of thecable.

In order to prevent its phenomenon of the sulfide trees, vari ousmeasures have been proposed. In one such measure, the cable sheath mayfurther be covered with a metal, which is strongly resistant against thewater-soluble sulfide, for instance, lead, to prevent the penetration ofsaid sulfide into the electric cable. In a second measure, a laminatetape consisting of a polyolefin film and an aluminum foil may be appliedalong or wound on the periphery of the insulated cable core, so that thepenetration of the water-soluble sulfide such as hydrogen sulfide isstopped by the metal layer interposed between the insulated cable coreand the cable sheath.

These measures are indeed very effective. However, the former measure isonly justified if it is known that there is water-soluble sulfide in theplace where the cable is laid. Ifthe cable is to be laid in places wherethe presence of the watersoluble sulfide is not prediatable, thismeasure is not advantageous because the metal sheath increases the costand weight of the cable. The latter measure is also not economicalbecause of the provision of the laminate tape. Besides, it is possiblethat during long period of use the water-soluble sulfide will permeatethrough the seam between adjacent tape edges to give rise to the growthof the sulfide trees. Thus, the second measure is not so effective inspite of the increased steps and cost of manufacture of the cable.

The invention, accordingly, is intended from the foregoing aspects, andit has an object of providing a polyolefinseries resin insulatedelectric cable, which is simple in construction and capable ofpreventing the growth of sulfide trees over a long period of use.

The present invention is based on the following finding: Inpolyolefin-series resin insulated electric cables provided with aplastic sheath outside the insulated cable core comprising an insulatinglayer consisting of polyolefin-series resin applied directly or with theaid of other insulating layer on the copper conductor or, if necessary,around a desired number of insulated cores, stranded together, it wasfound that hydrogen sulfide, ammonium sulfide, etc. penetrating into thecable from outside could be trapped in a sulfide-capture layer in theform of water-insoluble metal sulfides and that the sulfide tree couldbe prevented in the polyolefin-series insulating layer, by providing asulfide-capture layer of polyolefin-series resin composition containingsuch powder of metals, salts of the metals, or the mixture thereof as toform waterinsoluble metal sulfides by reaction with water-solublesulfide, in a desired position that is on the inside, outside, or bothsides of said polyolefin-series resin insulating layer or around adesired number of insulated cores stranded together.

The aforesaid sulfide-capture layer made of a polyolefin-series resincomposition containing metals or metal salts capable of reacting withthe water-soluble sulfide such as hydrogen sulfide to produce awater-insoluble metal sulfide may concurrently serve as a semiconductinglayer usually provided on the outer or inner side or on both sides ofthe polyolefin-series resin insulating layer, or it may be providedseparately from the semiconducting layer. When the sulfide-capture layeris provided on the outer side of the polyolefin-series resin insulatinglayer, it may concurrently serve as the protective sheath layer. In3-core electric cables, for example, the jute filler beneath theprotective sheath may be replaced with the polyolefin-series resincomposition containing metals or metal salts according to the invention.Thus, the end of preventing the growth of the sulfide trees can beachieved by providing the sulfide-capture layer around the copperconductor of the polyolefin-series resin insulated electric cable.

To this end, in accordance with the invention it is preferable that ametals or metal salts to be incorporated into the sulfidecapturepolyolefin-series resin layer are water-insoluble and that themetalsulfide that will be produced by the reaction of metals or metal saltswith the water-soluble sulfide such as hydrogen sulfide are-also waterinsoluble. The above metals may be zinc, cadmium, silver, cobalt,strontium, bismuth, gold, tin, iron, copper, lead, nickel, antimony,manganese, vanadium, tellurium, and the above metal salts may be oxides,hydroxides, sulfates, chlorides, nitrates, carbonates and aliphatic andaromatic organic acid salts of aforementioned metals. The grain size ofthese metals or metal salts should not be too large, and is preferablybelow 100 meshes. Among the aforementioned metal salts, such salts oflead as lead oxide [P- bO], lead hydroxide [Pb(OH) lead carbonate [PbCOlead nitrate [Pb(NO,-,) lead chloride [PbCl lead acetate [Pb(CH CO leadsulfate [PbSO,], lead chromate [Pb- CrO,], lead peroxide [PbO red lead[Pb O lead sesquiox ide [Pb o white lead [ZPbCO Pb(OI-I) lead stearate[Pb(C I-I =,O mono-basic lead acetate [PbgO(C2H3O2)2], basic leadsilicate [PbO-H O-ZPbSiO b], tribasic lead sulfate [3PbO- H OL dibasiclead phosphite [ZPbO-PbI-IPOfl H O], dibasic lead phthalate [2PbO'Pb(CI-l O,)], tribasic lead maleate [3PbO'Pb(C,H- O )H O], lead salicylate[Pb(C H(OH)CO2)2] and dibasic lead stearate [ZPbO- Pb(C1 l-l35COO)2,such salts of zinc as zinc oxide [ZnO], zinc hydroxide [Zn(OH) zincsulfate [ZnSO zinc chloride [ZnCl zinc carbonate [ZnCO zinc stearate[Zn(C11H;,5 C09 zinc laurate [Zn(C,,H CO and zinc ricinoleate[Zn(C,7H32(OH)CO2)2], and such salts of cadmium as cadmium oxide [CdO],cadmium hydroxide [Cd(OH) cadmium sulfate [CdSO cadmium chloride [CdClcadmium carbonate [CdCO cadmium stearate [Cd(C,,l-I CO cadmium laurate[Cd(C I-I, CO and cadmium ricinoleate [Cd(C H (OH)CO2)2] areparticularly preferable as they are readily miscible withpolyolefin-series resin, inexpensive and readily available.

In accordance with the invention, the polyolefin-series resin, to whichthe aforementioned powdery metals or metal salts are to be incorporated,may include high density polyethylene, medium density polyethylene, lowdensity polyethylene, ethylene-vinylacetate copolymer,ethyleneethylacrylate copolymer, ionomer, isotactic polypropylene andisotactic polybutene-l. These polyolefin-series resins are suitable forthey are inherently excellent in water proofness, and less likely toundergo the degradation of physical properties even when theaforementioned powdery metals or metal salts are added thereto.

The invention will now be described by having reference to theaccompanying drawings, in which:

FIG. 1 is a sectional view of a polyethylene insulated electric cableprovided with a sulfide-capture layer, which contains a metal or a metalsalt capable of reacting with the watersoluble sulfide to produce awater-insoluble metal sulfide between a copper conductor and apolyethylene insulating layer, embodying the invention;

FIG. 2 is a sectional view of a cross-linked polyethylene insulatedelectric cable provided with a sulfide-capture layer, which contains ametal or metal salt capable of reacting with the water-soluble sulfideto produce a water-insoluble metal sulfide, covering a cross-linkedpolyethylene insulating layer, embodying the invention; 7

FIG. 3 is a sectional view of a polyethylene insulated threecoreelectric cable provided with a sulfide-capture layer surrounding threepolyethylene insulated cable cores, embodying the invention; and

FIGS. 4 and 5 illustrate the sulfide-capture effects obtainable inaccordance with the invention.

In the embodiment of FIG. 1, a copper conductor 101 having a desireddiameter is covered with a sulfide-capture coating 102 consisting of apolyolefin-series resin composition containing the aforementioned metalsor metal salts. The sulfide-capture coating 102 is covered with apolyolefin-series resin insulating layer 103 having a desired thickness,which is in turn covered with an outennost plastic cable sheath 105.

In the embodiment of FIG. 2, the sulfide-capture coating 102 of apolyolefin-series resin composition containing the aforementioned metalsor metal salts nature is provided on the outer side of thepolyolefin-series resin insulating layer 103.

In the embodiment of FIG. 3, three insulated cable cores, each having acopper conductor 101 and a polyolefin-series resin insulating layer 103thereon, are stranded together with a filler 104. These stranded cablecores are covered with the sulfide-capture layer 102, which is in turncovered with the outermost protective plastic cable sheath 105.

In addition to the above embodiments, the aforementioned metals or metalsalts may be incorporated into the outermost protective plastic cablesheath 105 or the semiconductive layer usually provided on the outer orinner side of the usual insulating layer 103 so that the sulfide-capturesubstance-incorporated layer may also serve as the sulfide-capturecoating 102. It is particularly advantageous in manufacture and from thestandpoint of economy to incorporate metals or metal salts capable ofreaction with such water-soluble sulfide as hydrogen sulfide to producemetal sulfides in the semiconductive layer on the inner or outer side ofthe usual insulating layer of the electric cable. In any case, anexcellent sulfide trapping efiect can be expected from the addition ofthe aforementioned metals or a metal salts to any other layersurrounding the copper conductor than the polyolefin-series resininsulating layer 103.

In this invention, the lower limit of the amount of metals or metalsalts to be added to a polyolefin-series resin composition constitutingthe sulfide-capture coating depends upon the coefiicient of waterpermeability of the polyolefin-series resin composition. With apolyolefin-series resin composition whose water permeability coefficientat a temperature of 40 C. is below 20 l0 (gem/cm. sec. cm. Hg), forinstance, high, medium and low-density polyethylene and polypropylene,at least 1 part by weight of the above metals or metal salts should beadded to I00 parts by weight of the resin. With a resin having apermeability coefficient of above this value, the addition of at least10 parts by metal weight of the metals or metal salts to parts by weightof the resin is necessary. The dependence of the range of the amount ofthe metals or metal salts to be added to the polyolefin-series resincomposition constituting the sulfide-capture coating upon the waterpermeability of the resin used stems from the fact that the substantialproportion of the accidents due to the penetration of the water-solublesulfide have actually taken place where the cable is laid in water, withwater acting as the carrier of a sulfide. There is no upper limit of theamount of the above metals or metal salts to be added so long as theaforementioned end alone is taken into consideration. However, if morethan 100 parts by metal weight of the above metals or metal salts areadded to 100 weight parts of the polyolefin-series resin used, thedegradation of physical and chemical properties of the resin becomesoutstanding depending upon the kind of the resin. For this reason, thepreferable range of the amount of the above metals or metal salts ispractically less than weight parts, and more practically 5 to 60 partsby metal weight, with respect to 100 parts by weight of the resin.

The effects according to the invention will become more apparent fromthe results of experiments given below.

EXPERIMENT 1 Sample rods 10 mm. in diameter and 100 mm. in length weremade from compositions listed in Table 1 below. These sample rods wereimmersed either in the aqueous solution of ammonium sulfide or in thesaturated aqueous solution of hydrogen sulfide for a certain time. Aftereach of the successive time intervals they were drawn out of thesolutions and radially severed to determine the sulfide penetrationspeed by measuring the thickness of the portion blackened due toformation of lead sulfide. The smaller the thickness of the blackenedportion, the greater, it was judged, the sulfide trapping efiect. Theresults of experiments using the aqueous solution of ammonium sulfideare shown in FIG. 4, and those using the saturated aqueous solution ofhydrogen sulfide are in FIG. 5.

As is apparent from FIGS. 4 and 5, so far as the effect of trapping thewater-soluble sulfide is concerned, it is increased by higher content ofthe metal or metal salt additive capable of reacting with thewater-soluble sulfide to produce a metal sulfide.

EXPERIMENT 2 The results of experiments conducted about the effect ofadding the metals or metal salts capable of trapping the sulfide to thebase resin on the processibility and mechnical properties of theresultant composition are given in Table 2 below.

TABLE 2 Additive Roll Sulfide content kneading Tensile capture (inweight process strength Elongation Brittle Base resin additive parts)ability (kg/mm (percent) temperature 0 Good 1.67 698 Below 7(l C. 30 do1.60 636 Below -05 C. 50 ..(lo 1.02 650 Below -60 C. 150 Fairly good.1.15 310 Below 20 C.

EXPERIMENT 3 Sample sheets were made of polyolefin-series resincompositions shown in Table 3 below.

determine the content of sulfide-capture additives, i.e., metals or metlsalts, according to their kinds as well as the kind of the base resin inwhich they are used.

I Fine powder. 2 N one.

Fifty-two sample pieces were prepared, each measuring 1.0- mm.-thick,40-mm.-wide, and 200-mm.-long and composed of a copper plate,O.5-mm.-thick, -mm.-wide, and 160mm.- long, completely covered byhot-pressing with different kinds of polyolefin-series resincomposition. These sample pieces were then immersed in the saturatedaqueous solution of hydrogen sulfide. After days of immersion, eachcopper plate was stripped of its resin covering and checked forblackening due to formation of copper sulfide. ln Samples Nos. 24, 33,41, 44, 48 and 52, whose covering did not contain any powdery metal ormetal salt, remarkable blackening of the copper plate was observed.

As is apparent from the above three kinds of experiments, for formationof the sulfidecapture layer, it is desirable to Some examples of thefabrication of the polyolefin-series resin insulated electric cableaccording to the invention will now be described.

EXAMPLE 1 parts of white lead, which was in turn covered with apolyethylene sheath, l.5-mm.-thick, to complete a polyethylene insulatedelectric cable. As a comparison sample, an electric cable similar to theabove, but free from white lead was also produced.

These electric cables were then immersed for 245 days in an aqueoussolution of ammonium sulfide held at room temperature. Then, they weretaken out of the solution, and their coverings were removed off theconductor. In the cable according to the invention, the copper conductorwas found blackened only in a minor degree, and the formation of leadsulfide was observed in the conductive polyethylene layer, but nosulfide trees were observed in the polyethylene insulating layer. On theother hand, in the comparison cable the copper conductor was observed tobe blackened, and tree-like crystals of copper sulfide about 0.6 mmJlongwere seen growing in thepolyethylene insulating layer.

EXAMPLE 2 A copper conductor 22 sq. mm. in cross section wasextrusion-coated to a thickness of 1.2 mm. with a semiconductivepolyethylene composition consisting of 100 weight parts ofethylene-vinylacetate copolymer (with a vinylacetate content of 3percent), weight parts of carbon black and 20 weight parts of litharge,on which was then formed a cross-linked polyethylene insulating layer, 3mm. in thickness, which was in turn extrusion coated to a thickness of1.2 mm. with the semiconductive polyethylene composition consisting of100 weight parts of ethylene-vinylacetate copolymer (with a vinylacetatecontent of 3 percent), 20 weight parts of carbon black and 20 weightparts of litharge, and the resultant cable core was finally covered witha polyethylene sheath, 1.5 mm. in thickness to complete a polyethyleneinsulated electric cable. A comparison cable similar to the above, butfree from litharge was also produced.

These electric cables were immersed for 490 days in the saturatedaqueous solution of hydrogen sulfide held at a temperature of 50 C. Thenthey were taken out of the solution,

and their copper conductor was stripped of the coverings. In

the cable according to the invention, no copper sulfide crystals wereobserved in the cross-linked polyethylene insulating layer. On the otherhand, in the comparison cable extreme blacking of the copper conductorwas observed and treelike copper sulfide crystals, about 1.0 mm. long,were observed in the cross-linked polyethylene insulating layer.

EXAMPLE 3 Three insulated cable cores, each comprising a conductorconsisting of seven copper wires (0.8 mm. in diameter) stranded togetherand polyethylene (with density of 0.92 and melt index of 2.0) insulatinglayer, 0.8-mm.-thick, thereon were stranded together with jute. On theresultant strand was wound a cotton tape, which was then extrusioncoated to a thickness of 2.0 mm. with a polyethylene compositionconsisting of 100 weight parts of polyethylene, 2.5 weight parts ofcarbon black and 40 weight parts of white lead to complete a 3-corepolyethylene insulated control cable. A comparison cable similar to theabove free from white lead in the cable sheath was also produced.

These control cables were left immersed for 147 days in an ammoniumsulfide solution while being charged with 200 volts at room temperature.Thereafter, they were broken for examination. in the cable according tothe invention, no formation of sulfide copper crystals was recognized inthe polyethylene insulating layer. On the other hand, in the comparisoncable tree-like copper sulfide crystals about 0.4 mm. long, were seengrowing in the polyethylene insulation.

EXAMPLE 4 thus producing a model cable. A comparison cable similar tothe above, but free from zinc oxide in the polyethylene compositionpolypropylene insulating layer was produced.

These cables were then immersed for 6 months in the saturated aqueoussolution of hydrogen sulfide held at a temperature of 50 C. while ACvoltage of 400 volts was applied to them, thus causing forceddeterioration of them. Before the forced deterioration, their insulationresistance was 7.17x10 (nu -km). After the forced deterioration for 6months, the insulation resistance of the comparison cable decreased to3.90Xl0 (MO-km), whereas the insulation resistance of the cableaccording to the invention decreased merely to 2.45Xl0" (M.Q-km).Detailed examination of these deteriorated cables by breaking themrevealed that in the cable according to the invention the copperconductor had undergone only slight blackening, and the polypropyleneinsulating layer contiguous to the conductor had undergone no change. Onthe other hand, in the comparison cable a great deal of black coppersulfide crystals about 10p. long were seen growing in the polypropyleneinsulating layer.

EXAMPLE 5 Three insulated cable cores, each comprising a conductorconsisting of seven copper wires (0.8 mm. in diameter) stranded togetherand a O.8 -mm.-thick cross-linked polyethylene (with gel fraction of 78percent and density of 0.92) insulating layer thereon, were strandedtogether with jute. On the resultant strand was wound a cotton tape,which was then extrusion coated to a thickness of 1.5 mm. with apolyethylene composition consisting of weight parts of high densitypolyethylene (with desity of 0.96 and melt index of 0.2) and 45 weightparts of cadmium sulfate (CdSO which was in turn extrusion coated to athickness of 1.5 mm. with a polyethylene composition (with carbon blackcontent of 2.5 percent, density of 0.93 and melt index of 0.3) formingthe cable sheath, thus producing a model cable. A comparison cablesimilar to the above, but free from cadmium sulfate in the high densitypolyethylene composition was produced.

These cables were then immersed for 6 months in the saturated aqueoussolution of hydrogen sulfide held at a temperature of 50 C. while ACvoltage of 400 volts was applied to them, thus causing forceddeterioration of them. Detailed examination of these deteriorated cablesby breaking them revealed that in the cable according to the inventionthe copper conductor had sufiered no corrosion. On the other hand, inthe comparison cable a great deal of black copper sulfide crystals about70p. long were seen growing uniformly in the cross-linked polyethyleneinsulating layer.

As is apparent from the above examples, the sulfide-capture layer may beprovided between the copper conductor and the insulating layer thereon,or alternatively the sulfide-capture additives, which are powdery metalsor metal salts capable of reaction with hydrogen sulfide, etc. toproduce metal sulfides, may be added to a covering layer such as anouter semiconductive layer or protective plastic sheath layer.

It will be appreciated that according to the invention the end ofpreventing the dielectric breakdown of the polyolefinseries resininsulated electric cable (laid at places where they are affected bychemicals or on the sea-bottom) due to growth of sulfide trees in theinsulation resulting from the reaction of a water-soluble sulfide withthe copper conductor can be achieved by preventing the growth of thewater-insoluble copper sulfide crystals in the insulation throughaddition of metals or metal salts, which can actively react with thewater soluble sulfide entering the cable from outside such as hydrogensulfide, to produce a water-insoluble metal sulfide, to at least one ofthe covering layers surrounding the copper conductor other than thepolyolefin-series resin insulating layer. Thus, the above examples areby no meanslimitative, but various changes and modifications may be madewithout departing from the scope of the invention.

As has been described in the foregoing, according to the invcntion thesulfide-capture layer surrounding the copper conductor of thepolyolefin-series resin insulated electric cable and containing metalsor metal salts capable of reacting with the water-soluble sulfide toproduce a water-insoluble metal sulfide, completely captures thewater-soluble sulfide entering the cable from the outside and renders itinto a metal sulfide insoluble in water, so that the complete preventionof the growth of copper sulfide crystals, the so-called sulfide trees,constituting the leakage paths in the polyolefin-series resin insulatinglayer may be ensured to provide stable insulating characteristic of thecable insulation over a long period of use.

What we claim is:

l. The method of mitigating formation of sulfide trees in an insulatinglayer of an insulated electric cable having at least one insulated cablecore comprising a copper conductor encased within an insulating layer ofpolyolefin-series resin and a separate plastic sheath surrounding saidcable core which comprises providing said electric cable with a sulfidecapture layer separate from said insulating layer and said plasticsheath, said sulfide capture layer consisting essentially of a layer ofpolyolefin-seriesresin composition containing about to 150 parts byweight per 100 parts of resin of a substance or mixture of substancescapable of reacting with water-soluble sulfides to form water-insolublesulfides selected from the group consisting of powdered zinc, cadmium,silver, cobalt, strontium, bismuth, gold, tin, iron, copper, lead,nickel, antimony, manganese, vanadium and tellurium, and the oxides,hydroxides and salts thereof.

2. A method according to claim 1, wherein the material of saidpolyolefin-series resin insulating layer is selected from the groupconsisting of high-density polyethylene, medium-density polyethylene,low-density polyethylene and cross-linked polyethylene.

3. A method according to claim 1, wherein the number of said insulatedcable cores are at least three.

4. A method according to claim 1, wherein said sulfide-capture layersurrounds said insulated cable core.

5. A method according to claim 1, wherein said sulfide-capture layer iscontiguous to at least one side of said polyolefmseries resin insulatinglayer of said insulated cable core 6. A method according to claim 1,wherein the material of said sulfide-capture layer is of a compositionconsisting of parts by weight of at least one base resin selected fromthe group consisting of high-density polyethylene, medium-densitypolyethylene, low-density polyethylene, ethylenevinylacetate copolymer,ethylene-ethylacrylate copolymer, ionomer, isotactic polypropylene andisotactic polybutene-l and 5 to 60 parts by metal weight of at least onesubstance selected from the group consisting of powdery metals salts ofsaid metals, and the mixture thereof capable of reaction with thewater-soluble sulfide to produce a water-insoluble sulfide.

7. A method according to claim 1, wherein the material of saidsulfide-capture layer contains at least one substance selected from thegroup consisting of oxides, hydroxides, sulfates, chlorides, nitrates,carbonates and aliphatic and aromatic organic acid salts of lead, zinc,bismuth, cadmium, copper, iron and tin.

8. A method according to claim 1, wherein the material of saidsulfide-capture layer contains a lead salt selected from the groupconsisting of lead oxide, lead hydroxide, lead carbonate, leadnitrate,lead chloride, lead acetate, lead sulfate, Ie'ad'chromate, leadperioxide, red lead, lead sequioxide, white lead, lead stearate,monobasic lead acetate, basic lead silicate, tribasic lead sulfate,dibasic lead phosphite, dibasic lead phthalate, tribasic lead maleate,lead salicylate and dibasic lead stearate. v

9. A method according to claim 1, wherein the material of saidsulfide-capture layer contains a zinc salt selected from the groupconsisting of zinc oxide, zinc hydroxide, zinc sulfate, zinc chloride,zinc carbonate, zinc stearate, zinc laurate and zinc ricinoleate.

10. A method according to claim 1, wherein the material of saidsulfidecapture layer contains a cadmium salt selected from the groupconsisting of cadmium sulfate, cadmium chloride, cadmium carbonate,cadmium stearate, cadmium laurate and cadmium ricinoleate.

2. A method according to claim 1, wherein the material of saidpolyolefin-series resin insulating layer is selected from the groupconsisting of high-density polyethylene, medium-density polyethylene,low-density polyethylene and cross-linked polyethylene.
 3. A methodaccording to claim 1, wherein the number of said insulated cable coresare at least three.
 4. A method according to claim 1, wherein saidsulfide-capture layer surrounds said insulated cable core.
 5. A methodaccording to claim 1, wherein said sulfide-capture layer is contiguousto at least one side of said polyolefin-series resin insulating layer ofsaid insulated cable core.
 6. A method according to claim 1, wherein thematerial of said sulfide-capture layer is of a composition consisting of100 parts by weight of at least one base resin selected from the groupconsisting of high-density polyethylene, medium-density polyethylene,low-density polyethylene, ethylene-vinylacetate copolymer,ethylene-ethylacrylate copolymer, ionomer, isotactic polypropylene andisotactic polybutene-1 and 5 to 60 parts by metal weight of at least onesubstance selected from the group consisting of powdery metals salts ofsaid metals, and the mixture thereof capable of reaction with thewater-soluble sulfide to produce a water-insoluble sulfide.
 7. A methodaccording to claim 1, wherein the material of said sulfide-capture layercontains at least one substance selected from the group consisting ofoxides, hydroxides, sulfates, chlorides, nitrates, carbonates andaliphatic and aromatic organic acid salts of lead, zinc, bismuth,cadmium, copper, iron and tin.
 8. A method according to claim 1, whereinthe material of said sulfide-capture layer contains a lead salt selectedfrom the group consisting of lead oxide, lead hydroxide, lead carbonate,lead nitrate, lead chloride, lead acetate, lead sulfate, lead chromate,lead perioxide, red lead, lead sequioxide, white lead, lead stearate,monobasic lead acetate, basic lead silicate, tribasic lead sulfate,dibasic lead phosphite, dibasic lead phthalate, tribasic lead maleate,lead salicylate and dibasic lead stearate.
 9. A method according toclaim 1, wherein the material of said sulfide-capture layer contains azinc salt selected from the group consisting of zinc oxide, zinchydroxide, zinc sulfate, zinc chloride, zinc carbonate, zinc stearate,zinc laurate and zinc ricinoleate.
 10. A method according to claim 1,wherein the material of said sulfide-capture layer contains a cadmiumsalt selected from the group consisting of cadmium sulfate, cadmiumchloride, cadmium carbonate, cadmium stearate, cadmium laurate andcadmium ricinoleate.